As is known in the art, a variable valve timing mechanism, which varies the valve timing of engine valves (intake/exhaust valves) by varying the rotational phase of the corresponding camshaft relative to the crankshaft, has been used as a mechanism in an internal combustion engine mounted in a vehicle. The variable valve timing mechanism includes a case that rotates synchronously with the crankshaft, which is the output shaft of the engine, and a vane rotor that has a coaxial axis, is accommodated in the case in a relatively rotatable manner, and rotates synchronously with a camshaft of the engine. Accommodation chambers are formed in the case and accommodate vanes of the vane rotor. Each of the accommodation chambers is divided by a corresponding one of the vanes into an advancing hydraulic pressure chamber and a retarding hydraulic pressure chamber. The advancing hydraulic pressure chambers and the retarding hydraulic pressure chambers are controlled to rotate the vane rotor relative to the case. In this manner, the rotational phase of the camshaft relative to the crankshaft is varied.
In many cases, a variable valve timing mechanism having the above-described configuration includes a lock mechanism for locking the rotational phase of the vane rotor at a prescribed phase at the time when the engine is started. The lock mechanism locks the rotational phase of the vane rotor by engaging a lock pin projecting from the vane rotor with a lock hole formed in the case.
In some variable valve timing mechanisms, the locked rotational phase of the vane rotor, which is brought about by the lock mechanism, is set to an intermediate lock phase, which is at the middle of the rotational range of the vane rotor. FIG. 6 shows a front cross section of a variable valve timing mechanism with an intermediate lock mechanism, which locks the rotational phase of a vane rotor at the intermediate lock phase. As shown in the drawing, the variable valve timing mechanism has a vane rotor 51 having three vanes 50 and a housing 53 including three accommodation chambers 52 for accommodating the vanes 50. The housing 53 is fastened to a cam sprocket 54 and a cover 55 (see FIG. 7), which covers the front side of the housing 53 as viewed in the drawing, in an integrally rotatable manner. The housing 53, the cam sprocket 54, and the cover 55 form a case for receiving the vane rotor 51.
With reference to FIG. 6, a lock pin 56 for the intermediate lock mechanism is arranged in one of the vanes 50 of the vane rotor 51. As illustrated in FIG. 7, which shows the cross section of the variable valve timing mechanism taken along curve VII-VII of FIG. 6, when the vane rotor 51 is in the intermediate lock phase, the phase of the lock pin 56 coincides with the phase of a lock hole 57 formed in the cam sprocket 54. In this state, as the lock pin 56 projects toward the cam sprocket 54, the lock pin 56 becomes engaged with the lock hole 57, thus locking rotation of the vane rotor 51.
However, for the reason described below, it is not easy to reliably lock the relative rotational phase of the vane rotor 51 at the intermediate lock phase. Specifically, in many variable valve timing mechanisms with an intermediate lock mechanism, in order to simplify a hydraulic system, a hydraulic circuit for controlling the phase of the vane rotor 51 and a hydraulic circuit for operating the lock pin 56 are not formed independently from each other. That is, as illustrated in FIG. 8(a), the lock pin 56 is operated when the vane rotor 51 is advanced (the vanes 50 are). In this case, immediately after the phases of the lock pin 56 and the lock hole 57 coincide with each other, the lock pin 56 is pressed against a wall surface of the lock hole 57 at the advancing side, thus causing friction. This may hamper engagement of the lock pin 56 with the lock hole 57. In other words, the lock pin 56 is allowed to become engaged with the lock hole 57 only at the instant when the phase of the lock pin 56 and the phase of the lock hole 57 coincide with each other.
The variable valve timing mechanism with an intermediate lock mechanism has the following problem. Specifically, immediately after the engine is started, the intermediate lock mechanism does not receive hydraulic pressure. Accordingly, if there is space between the lock pin 56 and the lock hole 57, variation in the cam torque cause the vane rotor 51 to chatter, generating rattling noise. To avoid this, it is necessary to prevent the gap from being formed between the lock pin 56 and the lock hole 57. However, this requires a significantly high level of machining accuracy.
Conventionally, a variable valve timing mechanism with an intermediate lock mechanism including two lock pins, as described in Patent Document 1, has been proposed. As illustrated in FIG. 9, the variable valve timing mechanism has a vane rotor 61 having a plurality of (in the drawing, two) vanes 60, which project from the outer periphery of the vane rotor 61, and a housing 63 in which a plurality of accommodation chambers 62 are formed (in the drawing, two) to receive the vanes 60. The housing 63 of the variable valve timing mechanism has two lock pins 64, 65, which are spaced apart by a prescribed phase and capable of projecting toward the vane rotor 61. A lock groove 66, with which the two lock pins 64, 65 are engageable simultaneously, is formed in the outer periphery of the vane rotor 61.
FIGS. 10(a) to 10(c) illustrate the operating steps of the intermediate lock mechanism for the variable valve timing mechanism including the above-described two pins 64, 65. When the vane rotor 61 is rotated clockwise as viewed in the drawings with the lock pins 64, 65 disengaged from the lock groove 66, the lock pin 64 is first received in the lock groove 66 as illustrated in FIG. 10(a). As the vane rotor 61 is rotated counterclockwise continuously from the state of FIG. 10(a), the phase of the lock pin 65 coincides with the counterclockwise end of the lock groove 66 as viewed in the drawings and thus becomes engageable with the lock groove 66, referring to FIG. 10(b). In this state, the lock pin 64, which is engaged, is pressed against the clockwise end of the lock groove 66 as viewed in the drawing. However, the lock pin 65, which is not engaged, is maintained free. This allows the lock pin 65 to be smoothly received in the lock groove 66, as illustrated in FIG. 10(c). The vane rotor 61 is thus locked from rotating relative to the housing 63.
Since the variable valve timing mechanism includes the two lock pins 64, 65, as has been described, rotation of the vane rotor 51 is easily locked at the intermediate lock phase. However, in order to prevent chattering of the vane rotor 51 from occurring when the vane rotor 51 is locked in a state without hydraulic pressure, the lock groove 66 must be machined with significantly high accuracy, as in the case of the variable valve timing mechanism having the single lock pin 56.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-170085